1. Field of the Invention
The present invention relates to airspeed indicator apparatus and particularly to mach airspeed indicators capable of displaying mach and VMO from measured altitude and airspeed data.
2. Description of the Prior Art
Air data computers are well known in the aircraft industry which furnish a variety of information to the pilot. For example, U.S. Pat. No. 3,641,323 discloses a system employing an air data computer for displaying information indicative of deviations of the aircraft from the design point. In the disclosed system, the central computer is programmed with the profile representing optimum performance for the aircraft and stores several such profiles which are selectable for a given flight. These stored profiles of altitude versus mach number are used to determine the path of maximum operating efficiency for the aircraft. The computer senses the actual aircraft altitude and provides an output proportional to the programmed vehicle speed translated into mach number according to the selected stored profile in order to display the desired airspeed or mach number for optimum efficiency at a given altitude while also displaying the speed error in terms of mach number.
Another type of prior art air data computer is disclosed in U.S. Pat. No. 3,232,530, which discloses a mechanical type of air data computer which provides outputs which are functions of mach number directly from a motor which is caused to turn as a function of mach number. The motor which runs directly as a function of mach number is caused to respond to the difference between total pressure and static pressure and as such the system takes the measured pressure differential indicative of the difference between measured total pressure and static pressure to directly compute the mach number and display it.
Still another type of air data computer is disclosed in U.S. Pat. No. 3,843,877, which discloses the use of direct current-to-synchro and/or to-digital signal conversion for the sensed altitude signals. The disclosed system uses a source of signals proportional to mach number in order to compensate for the effects of mach number on the static source for the pressure altitude sensor.
Other types of prior art air data computer systems, by way of example, are disclosed in U.S. Pat. Nos. 4,163,387; 4,114,842; 4,039,165; and 4,047,001. U.S. Pat. No. 4,163,387 discloses the use of a microprocessor to display sensed altitude and other parameters to cabin passengers. U.S. Pat. No. 4,114,842 discloses the use of an air data computer in an acceleration limited preselect altitude capture and control system. U.S. Pat. No. 4,039,165 discloses a circuit for deriving a rate signal from an AC signal proportional to altitude displacement which is free from power supply and vertical gyro disturbances.
However, there are no prior art mach airspeed indicators known to applicant capable of internally computing VMO and/or mach number from measured altitude and airspeed for directly driving the VMO and/or mach displays, such as by closing a stepper servo motor servo loop for such a display by the internal computer.
With respect to such stepper servo motors, although their use is known in aircraft instrumentation, applicant is not aware of any such efficient low power stepper servo motor having a low inertia and a high step rate. Moreover, although permanent magnet stepper servo motors are well known, none are known to applicant in which a rare earth cobalt permanent magnet rotor is employed, nor one in which such a rare earth cobalt magnet is skewed so as to reduce the detent torque. This is so even though the use of rare earth cobalt magnets per se in high power large motors is known in which a high detent occurs, as is the use of skewed rotors per se which result in small torque.
Synchro-to-digital conversion is a well known technique, such as disclosed in U.S. Pat. Nos. 3,250,905; 3,832,707; 3,787,785; 3,071,324; 3,685,034; 3,744,050 and 3,737,885. Although some of these prior art techniques, such as disclosed in U.S. Pat. Nos. 3,737,885; 3,744,050 and 3,685,034, utilize multiplying digital-to-analog converters, they require separate converters for the sin and cos synchro functions. Thus, there are no synchro-to-digital converters known to applicant employing a single common multiplying digital-to-analog converter for synthesizing both sine and cosine.
These disadvantages of the prior art are overcome by the present invention.